The general idea is the art and science of photography depend on the laws of physics. But don't be too concerned; the Zone System does require good clear thinking and a lot of it, however it does not require a phD in the physical sciences. It does require logic and accuracy, so be aware that you will probably find the Zone System to be as intellectually rigorous as any method of photography you will encounter.

Actually that isn't always true. You might find methods and systems that deny access to their workings and appear to make things happen by "magic" to be much more difficult to understand.

Using the Zone System is rather like making biscuits, you know the ingredients (flour, buttermilk and shortening) it is the years of practice and finesse that make the difference. With the photographic medium you have to concern yourself with meter readings, Fstops, shutterspeeds, film speed and film development; the results of years of practice will speak for themselves in the faces of those that gaze upon your excellent photographs -- just as my mother must feel satisfaction from the satisfied faces of those of us sitting around her dinner table enjoying a rarely equaled buttermilk biscuit.

Here is the principle of the thing in a nutshell:

If your photographic system is calibrated
and
if the devices in that system behave consistently
then

• the light meter will tell you how much light to use to reproduce a subject as "middle gray".
• By using that knowledge carefully, you can not only place a subject on middle gray, you can place a subject on any gray that your paper can reproduce.
• In fact by careful use of exposure and film development, you can control not just one subject area but at least two subject areas.
• By more careful work, using filters and other techniques, you can sometimes control three or more subject areas, placing the combination of those areas an whichever parts of the grayscale you desire.

Controlling as many subject areas as feasible is a source of expressiveness that causes a well-conceived photgraph to spring to life. As a simple example, think of a cloud in a black and white print. Imagine the cloud against an uninteresting gray sky or against a dark brooding sky. Which has more raw impact? Of course, you may desire subtle interaction or some other effect on the viewer. Learning to control the medium is one way to open that door of expressiveness. Hmmm, how about a middle gray sky with slightly textured light gray cloud serving only as a momentary distraction to the viewer while you court her eye around your print.